How to Choose heat seal lidding film?

Multi-layered heat sealed lidding for rigid food containers

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Optimized tool designs and careful control of conduction-based direct contact constant heat sealing are not enough. Lidding materials, especially the heat seal layer, must be appropriate for the container and the food inside.

Find out more Soft Dairy and Spoonable Foods Heat Seal Overview looks at what it takes to package ready-to-eat soft dairy and spoonable foods in plastic cups. Soft Dairy and Spoonable Foods Lidding Sealing Challenges examines the challenges to getting consistently better and safer lidding seals. 
 

The right lidding material choices keep ready-to-use food safe and tasty

Rigid packaging lidding requires multiple laminated or coated layers. Each of these layers has a specific purpose. These layers consist of foil, paper, plastic or metalized plastic.

Soft dairy and spoonable food lidding must be relatively easy to open without shredding, especially for single-serve, ready-to-use foods like yogurt. In addition to being peelable, this lidding needs to provide a protective barrier and a strong seal that is tight enough to prevent leaks.

The critical roles of soft dairy rigid packaging and its sealed lidding

The simplest die-cut and rollstock lidding must provide these three essential functions:

• A surface for printed graphics
• A barrier to environmental exposure like water and oxygen
• A layer that heat seals the lidding to the rim of the cup or tub

Lidding Film Basics

The most basic type of lidding for ready-to-use soft dairy and spoonable foods has these layers:

• An outer substrate (backing) printable layer or one bonded to another printable layer above it
• One or more middle barrier layers
• Inner sealant layer

It is not unusual for soft dairy and spoonable food lidding films to have more than three layers. Each added layer serves a different purpose. Adhesive tie-layers combine two or more layers that would not otherwise adhere well to each other.

Typical 3-layer lidding film with an adhesive tie layer

Lidding Substrate Layers

Substrate layers are the foundation of lidding films

Substrate layers are the mechanical foundation for lidding. Other lidding layers bond to the substrate layer. Substrate layers are usually suitable for printing. But, if they aren't printable, they are bonded to a printable layer directly above them.

Lidding uses one of three different substrate materials. Any all-plastic lidding film uses a plastic substrate.

Basic types of lidding substrate:

• Foil
• Paper
• Plastic

Physical characteristics considered for lidding design and material choices:

• Thickness
• Heat resistance
• Color and clarity
• Custom barrier properties
• Materials and sustainability
• Chemical resistance and compatibility with food

Service conditions that need to be considered when choosing lidding:

• Temperatures the filled containers will be exposed to
• Vacuum and pressure resistance during transport and processing

Barrier Layers

All food lidding film and die-cut lidding require barrier layers
Good seals and an attractively printed, tough substrate are not enough. As mentioned earlier, heat sealable die-cut lidding and lidding films for soft dairy and related foods are multi-layer. Lidding always includes one or more barrier layers.

Barrier lidding layers for soft dairy and spoonable foods
Food products need to be protected from contamination to keep them fresh, tasty and safe to eat. While good lidding seals maintain product quality and protect consumers, the barrier layer extends shelf life and protects the sensory quality of food products.

What are lidding barrier layers?
Barrier layers in food packaging prevent the transmission of oxygen, moisture and other contaminants. These environmental barriers help keep food fresh for longer periods and extend product shelf life. Blocking oxygen is especially important for protecting live culture products like yogurt.

Essential lidding barrier layer functions:

• Extend shelf life
• Prevent contamination

Cups, pots, tubs and their lidding provide barriers to:

• Light
• Vapor
• Grease
• Moisture
• Chemicals

Barrier layers also keep desirable odors and flavors in and undesirable aromas and flavors out.

Heat Seal Layers

What lidding heat seal layers provide for both die-cut and plastic lidding film:

• Peelability
• Hot tack strength
• Heat seal strength
• Known and consistent seal initiation temperatures and operating windows

Seal layer initiation temperature and good seals

What is the heat seal layer initiation temperature?
Two temperature specifications apply to all lidding heat seal layers. One is the seal initiation temperature (SIT), while the other is the heat seal operating window. The SIT temperature is the lowest temperature where the sealing layer creates a strong seal against the container rim.

The heat seal operating window is the temperature range between the SIT on the low end and the highest temperature the heat seal layer can withstand and still form a strong seal.

What is the heat seal or sealant layer for rigid packaging lidding?

Sealant layers are coated onto, laminated onto or extruded onto lidding and are the final or bottom layer of multi-layer lidding. Sealant layers allow the lidding to be heat sealed to the rims of preformed and thermoformed cups, pots, tubs and trays.

There are two basic types of heat seal layers used for soft dairy and spoonable food lidding:

• Plastic heat-activated heat seal layers or films
• Heat-activated heat seal lacquer coatings

Heat seal layers are polymer-based heat-activated adhesives (hot melts). They are usually added using extruded or laminated film layers or solvent-based liquid coatings (lacquers). In general, the mechanics of the heat sealing process for both types are the same.

Beyond the two basic types of heat seal layers, there are other issues to consider when selecting a lidding and its heat seal layer:

• The container plastic the sealing layer must bond to
• The lidding film material (substrate) that the heat seal layer bonds to
• The food processing, preparation and handling conditions of the packaging line

For soft dairy and similar foods, these are service conditions that need to be considered when choosing the heat seal coating or layer and the seal it creates:

• Chemical resistance to the food product
• Vacuum resistance during transport and processing
• Temperatures the filled containers will be exposed to
• Compliance with FDA requirements for food contact materials

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Die-cut Lidding and Rollstock Lidding

Soft dairy packaging uses lidding that comes as either loose pre-cut die-cut lids or continuous lidding rolls called rollstock. Both types of rigid packaging lidding are designed to be heat sealed onto the rim of the food containers.

About rollstock lidding and lidding films

Rollstock lidding and the lidding rolls used to create die-cut lids are types of flexible packaging. Companies that make multi-layer flexible packaging, including lidding, are known as converters.

Converters are experts at modifying and combining basic materials to generate wide, long, and massively big rolls of flexible film and lidding. The most common production methods they use are coating, laminating, and printing. They use paper, polymers (plastics), metalized plastic or foil for the different lidding layers.

After producing these large industrial-sized rolls, they are slit, printed and rolled into more manageable sizes for automated food processing equipment. These smaller, narrower rolls or rollstock are used directly on packaging machinery or pre-cut into single die-cut lids.

About die-cut lidding and foil

Aluminum foil layers are common in die-cut lidding used for soft dairy and similar meals. This popularity is because it is an excellent barrier to moisture, light and oxygen.

The foil used in lidding must be coated or laminated on both sides using adhesive tie layers to bond to the other lidding layers. The foil is protected from punctures and tears by a top or outward-facing layer. This protective layer is usually printed or bonded to a printable layer above it.

Foil die-cut lidding
The simplest foil die-cut lids are made from aluminum with a heat-activated lacquer coating, or plastic layer added that is heat sealed to the container rim.

Plastic die-cut lidding
Plastic die-cut lids are composed of multi-layer plastic, like flexible packaging film rollstock. Polyester (PET) plastic is the most typically used plastic for die-cut lidding.

Paper die-cut lidding
Paper die-cut lids are made from a combination of paper-laminated aluminum and plastic film.

Plastic lidding film advantages versus foil lidding:

Foil lidding has exceptional built-in barrier properties, but plastic-only lidding films have distinct and attractive benefits.

Here are some of the benefits of using multi-layer plastic lidding films instead of die-cut foil lidding:

• Printing plastic film tends to be cheaper than printing die-cut foil
• Multi-layer films tend to be more technically challenging to make
• Plastic lidding film tends to be more tear and puncture resistant than foil
• Plastic lidding allows the use of metal detection to help ensure food safety
• Plastic lidding film tends to be more economical for higher volume production
• Mono-material or mono-polymer plastic packaging films are more easily recycled
• Tear and puncture-resistant lidding films reduce the need for caps or overcapping
• Thinner plastic lidding films means longer rolls which reduces lidding roll changeover frequency
• Mono-material film and cup combinations mean scrap plastic is reusable and has a practical market value

Peelability

What is peelability?
One of the most critical factors in choosing lidding heat seal layers for their performance is the peel seal or peelability desired in the packaged product. The lidding on single-use dairy packages is designed to peel away from cup rims without shredding or damaging the container.

Peelability requires matching the proper lidding seal layer to the cup's material, as well as a well-controlled seal initiation temperature

The peelable lidding design challenge
One of the key challenges of designing peelable lidding seals is that it needs to maintain its seal even as the packages move through the food supply chain.

General ways you can categorize lidding peelability or peel strength:

• Easy peel
• Medium to strong peel
• Very strong peel

Soft, spoonable dairy and related food products tend to be packaged with medium to strong peel strength. This level of peelability protects them during shipping and handling but provides practical ease of use for consumers.

Why is peelability important?

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• It's consumer-friendly
• It provides evidence of tampering but is still easy to use

Peelability necessitates matching the lidding and heat seal layer to the container plastic and controlling the heat seal process. The ultimate result is a peelable seal that isn't too weak to fail and isn't welded and bother customers.

Sustainability recycling goals matter too

A recent issue affecting the design, production and use of lidding films is recyclability. As more manufacturers work to reduce their waste stream and increase their efficiency, market pressures increase the demand for recyclable lidding film solutions.

A growing trend in consumer-packaged dairy products is a mono-plastic, mono-polymer or mono-material lidding film and cup combination.

What is mono-material packaging?
It is important to note that increasingly soft and spoonable dairy packaged products use mono-material or mono-polymer flexible plastic lidding films. Mono-material films are films where all the layers of the lidding film are the same plastic-type as the cups they are sealing. This use of mono-materials makes the packaging easier to recycle. Easier to recycle means the cup and lid are recycle-ready for existing single-stream recycling.

What is single-stream recycling?
Mono-material rigid plastic and lidding combinations fit more readily into single-stream recycling systems. Single-stream recycling is a simple recyclable material collection process.

Single-stream recycling allows consumers to use one container to collect and recyclable materials. It makes recycling collection much simpler and more manageable.
 

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Summing it all up

Processing soft dairy and spoonable foods require choosing the right lidding and rigid container materials. In addition, food safety and quality require sturdy, protective seals between the lidding and the cup rims.

All of this comes together during the heat seal process. Optimizing tool design, control and sealing parameters provides consistent seals. The results are lower, predictable production costs and minimal scrap while maintaining production line speeds.

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Selected Reference Resources

• Analysis of Peelable Film in Food Packaging by Matt Baker >>
   
• Blown Film | Which Applications Are Most Suited for 7/9/11 Layers? Part 3 at PFFC, Paper, Film and Foil Converter >>
   
• ClearBags Barrier Packaging Explained at ClearBags >>
   
• Converters (Industry) at Wikipedia >>
   
• FDA Compliant, Food Grade and Food Safe blog post from ISM >>
   
• Flexible Laminates Within the Circular Economy by Ulphard Thoden van Velzen, Lisanne de Weert and Karin Molenveld >>
   
• Heat Sealing Considerations for Sterile Barrier Systems at DuPont™ Tyvek® >>
   
• Identification of polybutene-1 (PB-1) in easy peel polymer structures at Science Direct >>
   
• Medical Packaging: A Comparison of Heat Seal Coating and Peelable Film Technologies at MDDI, Medical Device and Diagnostic Industry >>
   
• Multi-layered packaging at Wikipedia >>
   
• Sealing Technology: The Overlooked Backbone of a Successful Production Line at Food Manufacturing >>
   
• Toray Plastics (America) Launches Lumilid® White Lidding Film for Refrigerated and Frozen Dairy Foods >>

Our blog post series about heat sealing packaging for soft dairy and spoonable foods 

• Soft Dairy and Spoonable Foods Heat Seal Overview >> 
  Learn what it takes to package ready-to-eat soft dairy and spoonable foods in plastic cups with secure, attractive and easily peelable lidding. Rigid plastic containers with heat-sealed flexible lidding require thoughtful design and optimized processing. 
   
• Heat Seal Lidding Materials for Soft Dairy and Spoonable Foods >> 
  Find out about the material decisions behind peelable soft dairy lidding. Fresh foods in rigid packaging need to be protected yet easy to access. Get a better understanding of how different lidding layers work together to protect the food inside. 
   
• Soft Dairy and Spoonable Foods Lidding Sealing Challenges >> 
  Get consistently better and safer lidding seals by knowing more about the challenges. The best outcomes start with optimized tooling and sealing control. 

Lidding applications

Dry food

Dry food products, such as coffee lightener or powdered drink mix, place minimal demands on the peelable heat seal coating, Dry food constructions include

Paper / XLDPE / HS – HIPS

Paper / adhesive / OPP / HS – PVC

X = EXTRUDED

For this end use, a water based EVA dispersion would function quite well. There is no toluene to absorb into the LDPE, and the EVA provides an easily peelable bond to the cup stock. An EVA provides an easily peeable bond to the cup stock. An EVA coating can be applied by any method and requires a relatively low drying temperature that will not distort the polyolefin film. FDA compliance on paper and / or polyolefin is also assured with this type of coating.

Diary Creamer

There are several types of dairy creamer lid stock. The simplest is

Paper / XLDPE / HS – HIPS

For this application, a moisture – resistant, peelable seal is required. An EVA – based gel lacquer is one of the most widely used heat seals. Gel lacquers are a solvent solution of a hot melt. The most common method of application is by directly heated gravure. Here again, a coating with direct food contact compliance on polyolefin film is required. A more sophisticated creamer lid structure is

Paper / XLDPE / AF / HS – HIPS or

Paper / XLDPE / AF / primer / HS – HIPS

An EVA based gel lacquer works well for such applications, but it sometimes bridges or membranes when it is peeled. One such case, a vinyl primer and heat seal must comply with FDA regulation 21CFR175.300.

Jelly and condiments

Jellies, jams. Pancake syrup, fast food sauces are often filled at 170-200 f. Sauces are often acidic and contain aggressive components such as mustard oil. The heat seals used to hold these products must have good hot tack at 200 F to prevent leaking lids, good moisture resistance, moderate grease resistance, and good chemical resistance. One type of construction is

PEST / adhesive / AF / primer / HS – HIPSPVC

Here the heat seal can be a vinyl or vinyl – acrylic applied over a vinyl primer. The primer is important to protect the foil from corrosion by the product. Another approach is

AF / adhesive / PEST / HS – HIPS or PVDC / HIPS PVC PP HDPE

A reinforced EVA gel lacquer will provide good adhesion to the PEST lid substrate as well as good salability to a broad range of cup stocks and good heat resistance for hot fill. Compliance with FDA regulation 21CFR177. is an important factor in these applications. Lidding for fresh and frozen citrus juices is a large and important market. The most common construction is

            Print / AF primer / HS – PVC  

Where the primer and heat seal are both solvent based vinyls. A pinhole free coating is important to protect the foil from the corrosive citrus juices. Low temperature adhesion is a must for frozen products. Both must comply with FDA regulation 21CFR175.300 

Greasy or oily foods

Lidding on a clear substrate for greasy foods such as cheese or peanut butter is a challenge. An often used construction is

OPP / print / PEOT / HE – PETC PVC

This heat seal coating is a solvent based EVA / nitrocellulose hybrid with excellent film clarity and grease resistance. It complies with FDA regulation 21CFR177.

Dual Ovenable trays

Ovenable trays are either PET or SBS board / PET in which food production are either microwaved or heated in a thermal oven up to 400 F. The cover can be

PEST / HS – PET or

SBS / PET / HS – PET

Where SBS = solid bleached sulfite

The heat seal for the film lid mist be clear, have heat resistance to 400 F, and still be peelable after heating. A board lid heat seal need not be clear, but it must have all the other attributes. Polyester – based coatings that comply with FDA regulation 21CFR177. are used in these applications.

Autoclavable and aseptic lidding

Lidding for autoclavable and aseptic applications presents some unique problems. The heat – seal coating must have good adhesion to foil, and they must be sealable to poly propylene for steam sterilization. The coating must also be stable to radiation sterilization. The construction is

AF / HS –PP

And the heat seal is a specially reinforced EVA gel lacquer, which provides a controlled peel. This type of package is used for medical applications, such as sterile – water and hydrogen – peroxide – sterilized aseptic packaging. The FDA regulation for the latter is 21CFR178.

Retorable Lidding

Few packaging concepts can match restorable lidding for extreme performance requirements. A retortable lid must withstand 121 C, 15 psi steam for 80 min without losing seal integrity and yet remain peeable after long term shelf storage. The special heat seal coating that can function in this environment is a modified polypropylene dispersion known as Morprime. A typical lidding construction could be

0.5 mil PEST / 1 mil AF / Morprime – PP or 2 mil AF / Morprime – PP

This coating is also unique in its selectivity. It must be applied to a metal substrate, such as aluminum foil, and requires drying / curing temperatures of 400 F for 30 – 35 seconds. It is sealable to either cast or molded polypropylene or to another Morprime surface at 450 F for 1 second. Typical food products packaged in retortable cups including apple sauce, orange juice, yogurt and cheese food spreads. Moprime complies with FDA regulation 21CFR175.300

Medical packaging  

Heat – seal coating for lidding of disposable packages for medical devices are applied onto Tyvek or surgical kraft paper, which is then sealed to several different plastic tray materials. Typical constructions are

Tyvek / primer / HS – PET HIPS HDPE PP

The heat seal coating must have adequate porosity to allow efficient entry and removal or ethylene oxide gas during sterilization, as well as good humidity resistance and sufficient heat resistance to overcome “sterilizer creep” When peeled from the tray, the coating should transfer completely from the lid to the tray flange to show evidence of closure. Specially formulated EVA dispersions have been successful in this application. The large particle size of the dispersion provides good porosity, and the EVA polymer provides a good peelable bond to almost all of the commercially used tray stocks. These EVA coatings are also stable under radiation sterilization. FDA compliance for direct food contact on any substrate makes these waterborne coatings suitable for lidding or medical package. Although this survey of heat seal coatings for lidding has covered a variety of materials, applications, and usage parameters, it is by no means exhaustive. As new lidding substrates and containers materials become available, new heat seal coatings will be required to meet the needs of the converters and end-users

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